Interest and research activity in the photoinitiated cationic crosslinking polymerizations of multifunctional epoxide and oxetanes monomers have increased rapidly as this technology has found broad use in many industrial applications. Decorative and protective coatings, printing inks and adhesives are just a few examples of those applications in which photoinitiated cationic polymerizations have experienced the most commercial growth. There are several major motivating factors driving the adoption of this technology. First, the ability to conduct these crosslinking polymerizations very rapidly, with low energy and without the use of an inert atmosphere provides important economic incentives. Second, since solvents are not employed, there are no emissions and consequently, the environmental consequences of these polymerizations are minimal. Lastly, the thermal, mechanical, chemical resistance, low shrinkage and adhesion characteristics of the network polymers that are formed are excellent. The industrial impact of photoinitiated polymerizations in general and photoinitiated cationic polymerizations in particular, is predicted to increase markedly in the future as this technology undergoes further maturation.
Early work in the field of cationic photopolymerization took advantage of commercially available epoxide monomers. These epoxide monomers were and still are widely employed in thermally induced condensation polymerizations together with coreactants such as amines, anhydrides and thiols for such purposes as coatings, adhesives, potting and encapsulating resins. They were not intended for use in cationic ring-opening addition polymerizations. Consequently, such monomers are not optimally designed for nor in many cases do they possess sufficient purity for this purpose. As the uses of photoinitiated cationic polymerizations in advanced applications increase, in many cases, these epoxides no longer meet the required higher performance characteristics that are demanded. For these reasons, there has been a long-standing interest in this laboratory in the design and synthesis of novel epoxide monomers expressly for use in cationic photopolymerizations.
The synthesis of epoxy-functional siloxanes has been described in several publications (Crivello, J. V.; Lee, J. L. Polym. Mtls. Sci. and Eng. Preprints, 1989, 60, 217; ACS Symposium Series No. 417, C. E. Hoyle and J. F. Kinstle, editors 1989, p. 398; Crivello, J. V.; Lee, J. L. J. Polym. Sci., Polym. Chem. Ed, 1990, 28, 479; Crivello, J. V.; Lee, J. L. Proc. of the RADTECH '90 North America Conf., Chicago, Mar. 25, 1990, p. 432). The structures of two typical examples of this class of monomers, I and II, are shown below.

These di- and tetrafunctional monomers, respectively, bearing epoxycyclohexane groups display high rates of photoinitiated cationic polymerization. For example, the photoinitiated cationic ring-opening polymerization of I is faster by a factor of at least ten when compared to the commercially available 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate. This latter compound is considered a “high reactivity epoxide” and is currently employed in most cationic photopolymerizations where epoxide monomers are used.
However, while monomers such as I and II undergo efficient cationic ring-opening photopolymerization to give crosslinked materials with excellent thermal and chemical resistance, they produce hard, brittle, glass-like materials with little elongation and flexibility. Therefore, there remains a need for additional epoxy silicone monomers with different backbone structures, in order to expand the range of mechanical properties obtainable from this class of monomers.
It has been reported that the hydrosilation of α,ω-Si—H difunctional polydimethylsiloxanes containing up to four silicon atoms with vinyl compounds can be carried out under conditions that give essentially quantitative yields of the monosubstituted products. (See, for example, US 2002-0137870-A1.) This reaction is quite unique and has no parallel in carbon chemistry. For example, the condensation of TMDS with VCHO takes place in toluene or 1,4-dioxane at 65° C. in the presence of Wilkinson's catalyst (tristriphenylphosphinerhodium(I) chloride) to give 98% of the desired monoaddition product. In contrast, at higher temperatures, the reaction is indiscriminate and a mixture of IV, II and unreacted TMDS are obtained.
Similarly, monohydrosilated products are also obtained using 1,1,3,3,5,5-hexamethyltrisiloxane and 1,1,3,3,5,5,7,7-octamethyltetrasiloxane in greater than 94% yields.
A wide variety of highly reactive di- and multifunctional epoxy silicone monomers can be synthesized by the straightforward use of the hydrosilation reaction. For example, I can be prepared directly in virtually quantitative yield by the reaction shown in involving the addition of a 2:1 stoichiometric ratio of 4-vinyl-1-cyclohexene-1,2-epoxide (VCHO) to 1,1,3,3-tetramethyldisiloxane (TMDS) typically at a temperature above 100° C. in the presence of a platinum or rhodium catalyst.

The same reaction can be carried out with other α,ω-Si—H difunctional polydimethylsiloxanes to extend the length of the siloxane chain between the reactive epoxycyclohexane groups. In addition, a wide assortment of other epoxy functional vinyl compounds can also be employed in this hydrosilation reaction. However, obtaining the α,ω-Si—H difunctional polydimethylsiloxanes of higher molecular weight, that is, with chains containing more than two silicon atoms, with specific, well-defined chain lengths is both problematic and expensive. Typically, such materials are obtained as mixtures containing a broad distribution of chain lengths from the cohydrolysis of dimethyldichlorosilane and dimethylchlorosilane. The mixture is then fractionally distilled to isolate the species with the desired chain length. As the chain length of these materials increases, it becomes increasingly difficult to separate the different species present due to the similarity of their boiling points. For this reason, α,ω-Si—H difunctional oligomeric olydimethylsiloxanes containing more than two silicon atoms are generally prepared only in small laboratory quantities.
Several reports have appeared in the literature describing the conversion of Si—H bonds to siloxane groups by a dehydrodimerization reaction. Kennedy, et al. have described the condensation of Si—H containing cyclic siloxanes in the presence of water and a platinum catalyst to prepare amphiphilic networks composed of siloxanes and polyolefins (Kennedy, et al., Macromol. Symp. 172, 56-66 (2002). Kawakami et al have carried out the condensation of phenylsilane with water in the presence of platinum and other noble metal catalysts to obtain branched oligomeric resins (Seino, et al., Polymer Journal, 35, 197-202 (2003). In both cases, a complex mixture of siloxane products was obtained.